In recent years, as an approach for environmentalism, hybrid vehicles, electric vehicles, fuel cell vehicles and the like that can employ driving force obtained from a motor to travel are gaining attention. These vehicles have a secondary a battery, a capacitor or a similar power storage mechanism mounted therein to store electric power supplied to the motor. The battery, capacitor and the like generate heat as they are charged/discharged. Accordingly they need to be cooled.
Japanese Patent Laying-open No. 2005-184979 discloses a power supply device for a vehicle that can cool a battery efficiently to effectively liberate the battery from troubles associated with temperature, while reducing noise audible to a passenger in a cabin of the vehicle. As described in Japanese Patent Laying-open No. 2005-184979, the power supply device comprises a battery unit including a plurality of batteries, an air blowing fan blowing air to the battery unit to cool the batteries, a switching element controlling electric power supplied to the air blowing fan, and a control circuit that changes a fan duty repeatedly turning the switching element on and off periodically, as predetermined, to supply the air blowing fan with electric power as controlled by a width of a pulse of PWM (pulse width modulation). The control circuit detects the temperature of the batteries of the battery unit by a temperature sensor and in addition thereto detects a vehicular speed signal output from the vehicle having the power supply device mounted therein to detect the vehicle's speed. The control circuit uses both the batteries' temperature and the vehicle's speed as variables to modify a duty at which the switching element is turned on/off. If the battery unit's batteries rise in temperature or the vehicle is increased in speed, the switching element is turned on/off with an increased duty to allow the air blowing fan to receive more electric power.
The power supply device described in the publication modifies a duty, at which a switching element supplying an air blowing fan with electric power is turned on/off, in accordance with vehicular speed and engine speed. Thus when vehicular speed and engine speed are raised and larger noise is accordingly caused, the air blowing fan can rotate fast by increasing a duty at which the switching element supplying an air blowing fan with electric power is turned on/off. Thus the driver never hears as a noise the sound caused by the air blowing fan as it is operated. Thus a noise audible to the passenger(s) of the vehicle can be reduced while the batteries can be efficiently cooled and thus effectively liberated from troubles associated with temperature.
However, the cooling fan does not necessarily rotate at a constant number of revolutions with a constant duty (duty ratio). However, Japanese Patent Laying-open No. 2005-184979 neither discloses nor suggests such issue. A number of revolutions of the cooling fan thus is not controlled precisely.